1. Field of the Invention
The invention relates to a simultaneous biaxial stretching machine for thermoplastic film webs, said machine having two transport rails arranged on both borders of the film web, on which transport rails gripping devices for the gripping of the borders of the film web are guided, each coupled with one another by flexible connecting elements and having a roller moving mechanism, the gripping devices being guided at constant small distance in a preheating zone, divergently and at an increasing mutual distance within a stretching zone, and in parallel or convergently within a fixing zone.
2. Description of the Prior Art
For a simultaneous biaxial stretching, it is necessary to spread the gripping devices within the stretching zone in the latitude direction as well as in the longitudinal direction. The moving apart in the latitude direction does not offer any difficulties by appropriate arrangement of the rails. Numerous systems are known for the moving apart in the longitudinal direction. Due to the high tensile forces within the entire machine it is necessary to design these systems in such a manner that additional forces are kept to a minimum. Particularly interfering transverse forces are significant here, which occur in link chains and may amount up to a 17-fold amount of the longitudinal force.
A simultaneous biaxial stretching machine according to the German-PS No. 28 41 510 utilizes as connecting elements band-like elements, which in each case connect two adjacent gripping devices and are adapted to be wound up on a drum within each gripping device. The drum is operated via a self-locking worm gear. The drive wheel of the worm gear is actuated by means of a chain, which extends in parallel with the rail. A constant changing ratio of the distance between the gripping devices results in each case over the length of the chain. Due to the fact that the longitudinal force within a simultaneous biaxial stretching machine is in the order of 10 000N, it is necessary to use comparatively massive and solid bands, because the bands alone transmit the tensile force between adjacent gripping devices. Such massive bands require, for the winding up, a drum of considerable diameter. Also the worm gear cannot be reduced at will, if it shall resist permanently the forces occurring. On the other hand, a minimal distance of the gripping devices of 50 mm in the compressed condition of the gripping devices has proved to be useful. On the basis of this amount it is possible to provide, on the one hand, the gripping devices mechanically stable and, on the other hand, the distances of the gripping positions for the web are still tolerable even in the moved-apart condition of the gripping devices. It turned out that the drum and the worm gear for the band-like connecting elements cannot be reduced to this size so that in the case of the prior art apparatus the necessary minimum distances between the gripping devices cannot be reached under operating conditions.